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Transients in operational amplifier with exponential transfer characteristic of correction capacitor driver

Keywords:

N.N. Prokopenko – Dr. Sc. (Eng.), Professor, Head of Department «Information Systems and Radio Engineering», Don State Technical University (Rostov-on-Don)
E-mail: prokopenko@sssu.ru
A.R. Gaiduk – Dr. Sc. (Eng.), Professor, Department of Automatic Control Systems,
Southern Federal University (Rostov-on-Don – Taganrog)
E-mail: gaiduk_2003@mail.ru
A.V. Bugakova – Undergraduate, Department «Information Systems and Radio Engineering»,
Don State Technical University (Rostov-on-Don)
E-mail: annabugakova.1992@mail.ru


In the schemes of high-speed operational amplifiers (op-amp) the correction capacitor drivers (DCc) with exponential feed-through characteristic are used. Their main feature is the low current consumption in the static mode and the high speed of the recharging of the correction capacitor Cc on a large signal. The op-amp considered in this article contains the DCc and the classical buffer amplifier. The correction capacitor Cc and the equivalent resistance Ri in the high-impedance node ensure the stability of the circuit and form the low-signal frequency of the unity gain of the op-amp. All elements of the functional scheme of the op-amp, except the driver DCc, are linear and are described by differential and algebraic equations. Moreover, DCc has an exponential dependence of the output current from the input voltage. The presence of the exponential function in the differential equation describing the op-amp makes the solution of this equation very difficult and, judging from the reference literature, it is not known. Therefore, the solution is found under the condition that the correction circuit of the op-amp has a sufficiently large time constant. This allows us to neglect the linear term in the original differential equation. The analytic estimates of the transient’s time obtained in this paper are approximate, since the expansion of the logarithmic function in a series with the retention of only the first power of the variable was used in their derivation. It is shown that the transient of the op-amp with the exponential characteristic of the driver is a monotonically increasing. Practically, the formulas for the transient time give acceptable results for the typical driver schemes. Recommendations on the selection of the main parameters of the op-amp are given.

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